When measuring transmission quality such as loss and delay of packets transmitted in a packet network, a method called a loopback test is generally used. For example, a plurality of measurement packets are transmitted to a node, and the number of responses to the measurement packets from the node and the waiting times until the responses are received are measured. Hereinafter, this method is called active measurement.
On the other hand, it is known that packet loss and delay in a network greatly affect the transmission quality of voice communication and large size file copy in an IP network. For example, even when about 1% loss occurs in the network, the quality deteriorates significantly. Therefore, the loss rate must be measured with a precision of at least 1%. In order to measure the loss with such precision, from hundreds to thousands of measurement packets must be transmitted/received in active measurement. Therefore, there is a problem that conventional active measurement has no small effect on user traffic.
As a different method, there is a method called passive measurement. This method calculates a loss rate from an irregularity in the sequence numbers written in packets. For example, in the case of a TCP/IP packet, the loss rate can be calculated by using the TCP sequence number. The sequence numbers are also written in RTP (Real-Time Transport Protocol) generally used in voice communication, and, in the same way, the loss rate can be calculated by using the sequence numbers. Since this method, which is different from active measurement, does not transmit measurement packets, the method has characteristics of not affecting user packets very much even when a communication line is congested by user packets. However, in passive measurement, every time a user packet is received, connections must be classified into application levels such as TCP and RTP and the sequence numbers must be analyzed for each connection, although passive measurement does not affect user packets because passive measurement does not transmit/receive measurement packets. Therefore, passive measurement has a problem that a large storage capacity is required and processing workload is heavy.
On the other hand, since, in active measurement, only the measurement packets have to be processed, active measurement has a merit that processing of software/hardware is relatively simple, processing workload is light, and measurement is realized with a low cost. A method is desired which can measure the packet transmission quality with high accuracy, with not significantly affecting user packets, while taking advantage of the merits of active measurement.
Japanese Laid-open Patent Publication No. 2003-510918 discloses a technique for counting a loss of transmission frames in a cellular wireless network which is an asynchronous network.